Understanding the Camera Histogram

Article and Photography by Ron Bigelow

The camera histogram (see Figure 1) is a great tool. It is one of the advantages of shooting digital. The camera histogram allows a photographer to evaluate exposure. If the exposure is off, the photographer can reshoot and capture the image with a proper exposure. The camera histogram is also a great learning tool that accelerates the learning curve. New techniques or approaches can be tried, and the exposure results instantly evaluated. The photographer can immediately learn what works and what doesn't. Then, he can proceed to shoot accordingly.

However, as with many things digital, it takes a bit of understanding to properly use a camera histogram. Indiscriminate use of the camera histogram, without understanding how it operates, can actually result in incorrect exposure decisions.

What does the Camera Histogram Show

The camera histogram shown in Figure 1 is a brightness histogram that shows the brightness dynamic range of the captured image (i.e., the range from the darkest to the lightest tones in the image that maintain accurate detail) that results from the settings that were active in the camera at the time an image was taken (in addition to the camera brightness histogram, some cameras have separate camera RGB histograms, which will be covered later). Unfortunately, there are two hidden problems with the camera brightness histogram. First, the camera brightness histogram shows the dynamic range of an image that utilizes the settings that were active in the camera at the time that the image was taken. This is fine for those that shoot JPEG images. However, those that shoot raw may use different settings. Thus, the camera brightness histogram is not necessarily an accurate indicator of exposure for those that shoot raw. Second, the camera brightness histogram shows the brightness dynamic range. It is possible that the camera brightness histogram could show an acceptable exposure when, in reality, one or two of the color channels has been clipped resulting in a loss of detail.

Histograms and Raw

As stated in the paragraph above, the camera brightness histogram shows the dynamic range that results from the settings that were active in the camera at the time an image was taken. However, the dynamic range of a raw file is almost always greater than if the same image is produced with the camera settings (e.g., a JPEG image). Thus, the camera brightness histogram is a fairly accurate indicator of the dynamic range of an image that was shot in JPEG mode but not for an image that was shot as a raw file. The settings for an image that is converted from a raw file are established in the raw converter. This is one of the big advantages of raw. Usually, the settings used in the raw converter are different than the settings that were in the camera when the image was taken -- this generally results in a different dynamic range than that shown in the camera brightness histogram.

For instance, testing has shown that one of my digital cameras has a dynamic range of seven stops when shot in JPEG mode with the default settings. However, the same testing shows that a raw image from that camera, that is converted in a raw converter, can contain up to 9.67 stops of dynamic range. In other words, the camera brightness histogram will show a dynamic range that is 2.67 stops smaller than the dynamic range of the converted file. Further analysis shows that the converted file contains 0.67 stops more dynamic range in the highlights and two stops more dynamic range in the shadows than the JPEG file.

So, what should a photographer that shoots raw do? The obvious solution is for the photographer to learn the difference between the dynamic range shown in the camera brightness histogram and the dynamic range of the converted file. This can be done by taking several images and comparing the camera brightness histogram, for each image, to the histogram (in the image editing program) of the same image once it has been converted in the raw converter. Then, the photographer can make mental adjustments when photographing. For instance, whenever I shoot raw, I know that I will have an extra 0.67 stops of dynamic range in the highlights and two stops in the shadows compared to what I see in the camera brightness histogram. It is also possible to set the camera settings such that they produce the largest dynamic range possible (e.g., set the contrast setting as low as possible). This will reduce the difference between the dynamic range shown in the camera brightness histogram and the dynamic range of the converted file. Again, some testing will be required to compare the camera brightness histogram to the histogram of the converted file.

Histograms and Channels

Another problem that sometimes occurs with the camera brightness histogram is that it may show no indication of clipping, but one or two of the three color channels may actually be clipped to a small degree. This will cause a loss of detail. In addition to a camera brightness histogram, some cameras also have camera RGB histograms (see Figure 2). These histograms show the dynamic range for each of the three color channels. For those that have a camera with RGB histograms, this problem is not an issue as long as the photographer chooses to use the camera RGB histograms instead of the camera brightness histogram.

Figure 2: Camera RGB Histograms

Those photographers that have cameras with only a brightness histogram have three choices: 1) they can bracket their shots, 2) they can adjust the exposure to reduce the possibility of clipping in the channels (e.g., reduce the exposure to protect the highlights from clipping), or 3) they can expose as they normally would and run the risk of clipping the channels. Those that shoot raw with cameras that display only brightness histograms may have one other option. The extra dynamic range afforded by the raw format, over that shown in the camera brightness histogram, may reduce or eliminate the possibility of clipping one or two of the channels, in the image converted from the raw file, as long as the brightness histogram in the camera shows no signs of clipping.

One thing to keep in mind with respect to the camera RGB histograms is that they suffer from the same problem as the camera brightness histogram. The camera RGB histograms show the dynamic range, for each channel, that results from the settings that were active in the camera at the time an image was taken. Thus, the camera RGB histograms are fairly accurate indicators of the channel dynamic ranges of an image that was shot in JPEG mode but not for an image that was shot in raw.

What Does This All Mean

Those photographers that want the best exposures must learn how to interpret the camera histograms to properly set the exposure.